1. Field of the Invention
This invention relates to storage batteries of the lead-acid type. More particularly, the invention relates to methods for producing negative plates for such batteries.
Lead-acid storage batteries, commonly used in connection with automotive and other ignition systems, and in industrial applications, for example fork lift trucks and stand-by power systems, contain at least one cell which consists of positive electrodes of lead peroxide and negative electrodes of spongy lead, immersed in a sulfuric acid electrolyte. As these batteries are of the secondary type, they can be recharged at any point during the discharge portion of their charge/discharge cycle by applying an external current source to pass electrical current through the battery cells in a direction opposite to that in which the cells supply current to a load. Accordingly, lead-acid storage batteries can be continuously maintained at or near full electrical capacity by means of an alternator or generator connected thereto and operated, for example, by an associated engine.
2. Description of the Prior Art
The negative electrodes of lead-acid storage batteries, commonly called negative "plates," are usually fabricated by conventional techniques which include, for example, the steps of preparing a paste from lead oxide (usually litharge and/or red lead) and sulfuric acid, shaping the paste to a desired configuration by applying the paste to a supporting grid composed of a lead alloy having the desired size and shape, drying or "curing" the pasted grid by exposure thereof to an atmosphere containing carbon dioxide under controlled conditions of temperature and humidity, and then forming (charging) the cured plate in a sulfuric acid bath.
A typical paste for a negative plate of a lead-acid storage battery according to the prior art can be prepared by mixing lead oxide(s), usually in the form of litharge or red lead, with dilute sulfuric acid. Suitable lead oxides and mixtures thereof used for this purpose in the lead-acid storage battery industry are described in G. W. Vinal, Storage Batteries, 4th ed., at pages 21-23. The sulfuric acid usually comprises about 40-42 percent* of the paste; fibers (to facilitate handling of the plate after pasting) and water (to provide a paste having the desired consistency) are also usually added. FNT *The terms "percent" and "parts" as used herein and in the appended claims refer to percent and parts by weight unless otherwise indicated.
It has become common practice in the art to add relatively inert materials termed "expanders" to the paste from which negative plates are produced. The function of expanders is to prevent contraction and solidification of the spongy lead of the plate after its installation in the finished battery assembly and consequent diminution of the electrical capacity of the battery and shortening of its useful life. Compounds heretofore suggested for use as suitable negative plate expanders include, for example, lampblack, barium sulfate, graphite, ground carbon, ground wood, and insoluble organic compounds such a lignosulfonic acid and its derivatives. The effects and advantages of the use of expanders in producing negative battery plates is discussed in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. ed., vol. 3, at page 655.
U.S. Pat. No. 1,929,357 suggests blending cellulose or the like with barium sulfide and sodium sulfate to produce barium sulfate and sodium sulfide, and drying and pulverizing of this mixture. The dried and pulverized mixture is then added, to the extent of from 0.5 to 1.0 percent, to a mixture of litharge (98.75 to 99.25 percent) and lampblack (0.25 percent), and the resulting mixture is subsequently moistened with dilute sulfuric acid to produce a paste having a desired consistency. This patent also discloses that barium compounds such as barium hydrate and barium nitrate can be used in place of the barium sulfide and precipitated, for example, with sodium carbonate in lieu of sodium sulfate.
The negative plates of a lead-acid battery, composed of a mixture of lead sulfates when the battery is in a discharged condition, are converted to a sponge-like mass of lead particles when the battery is charged. These lead particles have a tendency to coalesce and, consequently, to reduce the sponge-like nature of the electrode. It is believed that expanders prevent such coalescence and enable the lead particles to remain soft and spongy, thus imparting improved electrical characteristics to the electrode.
An additional requirement in the manufacture of negative plates for lead-acid batteries is that they be "cured" after pasting. In this process the plate can be dried, either at ambient or elevated temperature, in a humid atmosphere containing a small percentage of carbon dioxide. The carbon dioxide reacts with the lead oxide(s) in the battery plate to form lead carbonate, which acts as a cement to hold the electrode together and to impart physical strength during subsequent processing and during operation of the finished battery. Without adequate curing, battery plates have been found to have insufficient strength to survive further processing or to provide long, trouble-free life in operation in the finished battery.